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When asked if mountains grow slowly and steadily versus in rapid spurts, most people intuitively gravitate to the “slow and steady” model. Mountains, we are taught, take an incomprehensively long time to build up their scads of boulders, jagged peaks and high-altitude plateaus.
In fact, most known mountain building processes do require large amounts of time to complete their skyward climb. But for every rule there is an exception. Consider the Himalaya and Andes mountains—despite their relative geologic youth, these mountain belts rank among the world’s tallest peaks. And therein lies the mountainous paradox: How do geologically young mountains grow extremely tall in extremely short time periods?
Conventional geology tells us that as the earth’s tectonic plates collide and dive beneath one another, and these actions cause the earth’s skin to crumple and fold. For a superficial visual effect, pinch together an inch or two of your forearm skin. Just as your skin crumples into peaks and valleys under pressure from your fingers, deforming tectonic pressures cause the earth’s crust to shorten and thicken into crenulations and folds, which alpinists yearn to climb and landscape photographers strive to capture on film. But below the surface, mountains have deep roots where dense material accumulates over time, often from the action of one tectonic plate diving beneath another whereby material is scraped off of one onto the other. It was previously thought that a gradual erosion of this root by the more plastic asthenosphere resulted in the gradual rise of the crust (see figure right).
But a new study tracking the uplift of a central portion of the massive Andes Mountains in South America shows that mountain building—what geologists term “orogeny”—may actually occur in much faster fits and spurts than previously realized due to the rapid loss of large amounts of material from the mountain’s root.
While conventional theory would predict that the Andes Mountains rose gradually and in sync with the scrunching of the Nazca plate beneath the South American plate, which scientists know has caused dense material to accumulate millennia after millennia up to 70 kilometers below South America’s western coast, Florida Museum of Natural History paleontologist Bruce MacFadden said that this is not what happened after all. MacFadden is a co-author of the study published June 6 in the journal Science.
“Instead of the Altiplano rising little by little each year, we found two phases of spasmodic or punctuated uplift interspersed by millions of years of stability,” MacFadden said.
The authors assert that as the crustal layer, or lithosphere (which floats above the mantle) was squeezed under deforming pressures, earth processes caused large parts of the accreted dense material to plummet downwards into the more plastic upper mantle layer, also known as the athenosphere. This loosening of the root load caused the surface crust layer to rise, buoyed upward like a released cork, by the excision of massive extra weight below.
“Our findings will force geologists to acknowledge that removal of lower lithosphere material could be an important process that causes rapid surface uplift in different mountain belts worldwide and over geologic time,” said lead author Carmala Garzione, a geologist at the University of Rochester. “The subduction process may cause shortening and thickening of the mantle lithosphere and dense lower crust that accumulates at depth until that dense material is removed rapidly—either by downward dripping, which is a convective process, or by another process called delamination.”
The researchers found that uplift began between 30 million and 20 million years ago, then leveled off into relative stability until “a pulse of rapid uplift” occurred between 10 million and 6 million years ago when the landscape rose between 1.5 and 3.5 kilometers in a massive upwards spurt. To reconstruct the Altiplano’s sequential rise, the researchers examined several lines of proxy evidence including two different types of stable isotopes, fossil plants and ancient magnetic-bearing deposits.
The researchers coaxed geochemical clues in the form of oxygen isotopes from ancient soil nodules made of calcium carbonate. The nodules were sampled from layered soil deposits between five million and 28 million years old. Oxygen isotopes serve as reliable proxy indicators for the actual temperatures in which they formed—so the researchers used them to reconstruct ancient temperature records, and then linked these records to known temperature clines associated with vertical elevation gain. They also analyzed magma and sediment as additional proxies.
“Carmie’s ability to put this study together shows her brilliance,” MacFadden said. “She’s synthesized research in theoretical geophysics, geochemistry, and paleontology and made a strong case for the timing and consequences of the Altiplano’s rise.”
A professor of earth and environmental sciences at Lehigh University who also researches ancient elevations said that while weaknesses were inherent when single proxy methods were used, the multiple methods used in this study made the results robust.
“Remarkably, the rapid recent uplift scenario presented here is similar to what I found for the Colorado Plateau,” Dork Sahagian said.
In 2005, Sahagian, who is also the director of Lehigh’s Environmental Initiative, organized a national workshop to refine and strengthen paleoelevation techniques. Garzione attended and presented her work, but Sahagian said her Andean project was just beginning at that time.
“The greatest novelty in their study is the number of proxies they brought to bear on the problem,” Sahagian said. “This is the right way to go about it.”
MacFadden, who has spent nearly three decades collecting and studying fossil mammals from Bolivia, contributed by leading the research team to several key fossil sites in the Altiplano where he had established geological age sequences in years prior. While Garzione’s interest was grounded in the geology, MacFadden’s interest in the project lay in understanding how the Andes’ birth affected South America’s ancient climate and animals. But in order for mountains to begin driving climatic changes, they have to reach a certain size.
“The big-picture question is: When did the Andes grow high enough to become drivers of the South American climatic regime? Because this event obviously had cascading effects upon plant and animal life across the continent,” MacFadden said.
Based on their findings, MacFadden said this likely happened around 10 million years ago.
Today, the massive Andes Mountain belt snakes 4,400 miles along the continent’s western edge and is the longest unbroken terrestrial chain on the planet, with peaks soaring to 22,841 feet. The world’s driest desert, the Atacama, stretches between the Andes’ central western foothills and the Pacific Ocean. Six hundred miles to the east, across the Bolivian bulge at the Andes’ widest point, the world’s largest collection of wetlands form the Pantanal.
“If we could rewind a video of the Andes’ formation,” MacFadden said, “we’d see how they grew into an immense force, affecting the distribution and abundance of moisture across large portions of South America.”
While we may not have a real-time video, we now have a much clearer picture of how the Andes climbed skyward in a geologically short amount of time, thanks to the efforts of Garzione, MacFadden and the rest of the study’s collaborators.
Additional study co-authors include: Gregory Hoke, University of Rochester; Julie Libarkin and Saunia Withers, Michigan State University; John Eiler, California Institute of Technology; Prosenjit Ghosh, Center for Atmospheric and Oceanic Science; and Andreas Mulch, Universität Hanover in Germany.
For more than 20 years, Florida Museum of Natural History Vertebrate Paleontology Curator Bruce MacFadden and his longtime South American colleague, paleontologist Federico Anaya of the Universidad Autónoma Tomás Frías in Potosi, Bolivia, have collected and studied fossils across Bolivia. They focused their efforts on a particularly specimen-rich area known as the Salla Beds, which tended to preserve many mammal species. (These deposits were also sampled for MacFadden’s most recent study on mountain building, see main story above.) Recently, an independent research team headed by Ralph Hitz of Tacoma Community College named two new genera of notoungulates—an extinct biological order of small- to medium-hoofed herbivorous mammals, endemic to South America—after MacFadden and Anaya, in honor of their extensive contributions to describing and studying the evolutionary history of South America’s ancient mammals. A description of both new genera, Brucemacfaddenia and Federicoanaya, was published in the Journal of Vertebrate Paleontology in May 2008.
Hitz said that when he was a graduate student in 1994, he went on a field expedition to the Salla Beds led by MacFadden, who had been working in the area since the early 80s. He said that though MacFadden hadn’t discovered the specific locality, it was due to his efforts that the “full relevance of this important fauna” came to be recognized. The oldest fossil primates known from South America were also found in the 25-million-year-old Salla Bed deposits.
“I was also impressed that he was engaged in the place of research as much as the objects of research,” Hitz said, “by which I mean he fully involved local institutions and paleontologists in the program, and also spent a considerable amount of time in La Paz teaching at the university.”
Hitz said he chose to also honor Anaya because of his perseverance and dedication to studying paleontology in Bolivia.
“Even in the U.S., paleontology can be a difficult career to practice. In Bolivia, it’s even more challenging,” Hitz said. “But Federico has continued through various obstacles. He has helped expand the science in Bolivia, and has also developed and maintained connections to researchers outside his country.”
While new species are often named after relevant scientists as an honorary gesture, it isn’t typically a common practice at the genus level, though it does occasionally happens—and sometimes even to non-scientists: Frank Zappa has a genus of goby fish, Zappa confluentus, from Papua New Guinea named after him. However, according to Florida Museum vertebrate paleontologist Richard Hulbert, a little more than a century ago an Argentinean paleontologist named Florentino Ameghino initiated a tradition of naming new South American ungulate genera after famous paleontologists and mammalogists—stringing together both their first name and surname. This practice—homenaje in Spanish, meaning to name something in honor of someone else—has resulted in the description of more than 30 new South American genera, using the compounded first name and surname of a relevant scientist, since the practice began in 1901, according to Hitz.
MacFadden said he was a little embarrassed when he learned of Brucemacfaddenia boliviensis, but other colleagues in his field say the gesture is fitting due to the breadth of his contributions.
“Dr. MacFadden devoted a considerable portion of his career working in Bolivia, leading very successful fossil-collecting expeditions there, studying and dating the rock layers producing the fossils, and assisting and training Bolivian scientists,” Hulbert said. “The most productive and significant region he worked in was the Salla Beds, which produced the fossils of Brucemacfaddenia. He and his co-workers also produced important information about Salla, including determining more precisely its geologic age and how it compared to similar faunas in Argentina.”